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          【JavaWeb】学习笔记之 Session 会话
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        <h3 id="什么是-Session会话？"><a href="#什么是-Session会话？" class="headerlink" title="什么是  Session会话？"></a>什么是  Session会话？</h3><ul>
<li>Session是一个接口</li>
<li>用来维护一个客户端和服务器之间关联的一种技术</li>
<li>每个客户端都有一个自己的 Session 会话<a id="more"></a></li>
<li>经常用于保存用户登录之后的信息</li>
<li>与 Cookie不同的是，Session是保存在服务器的，而Cookie保存在客户端</li>
</ul>
<h3 id="创建和获取："><a href="#创建和获取：" class="headerlink" title="创建和获取："></a>创建和获取：</h3><ul>
<li><p>创建和获取 Session 的 <code>API</code> 都是一样的</p>
<figure class="highlight abnf"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">req.getSession()<span class="comment">;</span></span><br></pre></td></tr></table></figure>

<p>第一次调用就是创建 Session 会话，之后调用就是获取已经创建好的 Session 对象。</p>
</li>
<li><p>判断 Session 是否是新建的</p>
<figure class="highlight isbl"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="title">isNew</span>();</span></span><br><span class="line"><span class="function">	<span class="variable"><span class="literal">true</span></span>: 表示刚刚创建的对象</span></span><br><span class="line"><span class="function">	<span class="variable"><span class="literal">false</span></span>: 表示获取之前创建的对象</span></span><br></pre></td></tr></table></figure>
</li>
<li><p>获取会话的唯一 ID 号</p>
<figure class="highlight abnf"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">getId()<span class="comment">;</span></span><br></pre></td></tr></table></figure>

</li>
</ul>
<h3 id="Session-域数据的存取"><a href="#Session-域数据的存取" class="headerlink" title="Session 域数据的存取"></a>Session 域数据的存取</h3><ul>
<li><p>保存数据</p>
<figure class="highlight less"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line"><span class="selector-tag">req</span><span class="selector-class">.getSession</span>()<span class="selector-class">.setAttribute</span>(<span class="string">"key"</span>,<span class="string">"value"</span>);</span><br></pre></td></tr></table></figure>
</li>
<li><p>获取数据</p>
<figure class="highlight less"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line"><span class="selector-tag">req</span><span class="selector-class">.getSession</span>()<span class="selector-class">.getAttribute</span>(<span class="string">"key"</span>);</span><br></pre></td></tr></table></figure>

</li>
</ul>
<h3 id="Session-生命周期控制"><a href="#Session-生命周期控制" class="headerlink" title="Session 生命周期控制"></a>Session 生命周期控制</h3><ul>
<li><p>设置 Session 的超时时长（以秒为单位）</p>
<p>负数的参数表示永不超时</p>
<figure class="highlight angelscript"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">public</span> <span class="built_in">void</span> setMaxInactiveInterval(<span class="built_in">int</span> <span class="built_in">int</span>erval)</span><br></pre></td></tr></table></figure>

<p>超过指定的时长， Session 就会被销毁。</p>
</li>
<li><p>获取 Session 的超时时间</p>
<figure class="highlight angelscript"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">public</span> <span class="built_in">int</span> getMaxInactiveInterval(<span class="built_in">int</span> <span class="built_in">int</span>erval)</span><br></pre></td></tr></table></figure>
</li>
<li><p>销毁当前 Session 会话</p>
<figure class="highlight actionscript"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">public</span> <span class="keyword">void</span> invalidate()</span><br></pre></td></tr></table></figure>
</li>
<li><p>Session 的默认超时时长</p>
<p>默认超时时长由服务器配置来决定的，例如在 Tomcat 服务器中的配置文件中设置默认超时时长就是30分钟</p>
<figure class="highlight xml"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line"><span class="tag">&lt;<span class="name">session-config</span>&gt;</span></span><br><span class="line">	<span class="tag">&lt;<span class="name">session-timeout</span>&gt;</span>30<span class="tag">&lt;/<span class="name">session-timeout</span>&gt;</span></span><br><span class="line"><span class="tag">&lt;/<span class="name">session-config</span>&gt;</span></span><br></pre></td></tr></table></figure>
</li>
<li><p>修改当前工程默认的 Session 超时时长</p>
<p>可以在当前工程的 <code>web.xml</code> 文件中进行配置更改，例如要修改为20分钟：</p>
<figure class="highlight xml"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line"><span class="tag">&lt;<span class="name">session-config</span>&gt;</span></span><br><span class="line">	<span class="tag">&lt;<span class="name">session-timeout</span>&gt;</span>30<span class="tag">&lt;/<span class="name">session-timeout</span>&gt;</span></span><br><span class="line"><span class="tag">&lt;/<span class="name">session-config</span>&gt;</span></span><br></pre></td></tr></table></figure>
</li>
<li><p>超时概念</p>
<p>Session 的超时是指客户端两次请求的最大间隔时长，只要客户端在 Session 的生命周期内向服务器发起请求，Session 的生命周期就会刷新。</p>
</li>
</ul>
<h3 id="Session-的底层实现原理"><a href="#Session-的底层实现原理" class="headerlink" title="Session  的底层实现原理"></a>Session  的底层实现原理</h3><p>Session 技术，其底层实现是基于 <code>Cookie</code> 技术进行实现的。因此在浏览器关闭之后，即使 <code>Session</code> 还未超时，再次访问时就会创建一个新的 <code>Session</code> 。这是由于 <code>Cookie</code> 在浏览器关闭之后会消失，进而导致 <code>Session</code> 的消失。</p>
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